Principal Buffer Policies

DTrace permits tracing in highly constrained contexts in the kernel.
In particular, DTrace permits tracing in contexts in which kernel software
may not reliably allocate memory. The consequence of this flexibility of context
is that there always exists a possibility that DTrace
will attempt to trace data when there isn't space available. DTrace must have
a policy to deal with such situations when they arise, but you might wish
to tune the policy based on the needs of a given experiment. Sometimes the
appropriate policy might be to discard the new data. Other times it might
be desirable to reuse the space containing the oldest recorded data to trace
new data. Most often, the desired policy is to minimize the likelihood of
running out of available space in the first place. To accommodate these varying
demands, DTrace supports several different buffer policies. This support is
implemented with the bufpolicy option, and can be set on
a per-consumer basis. See Chapter 16, Options and Tunables for more details on setting options.

switch Policy

By default, the
principal buffer has a switch buffer policy. Under this
policy, per-CPU buffers are allocated in pairs: one buffer is active and the
other buffer is inactive. When a DTrace consumer attempts to read a buffer,
the kernel firsts switches the inactive and active buffers.
Buffer switching is done in such a manner that there is no window in which
tracing data may be lost. Once the buffers are switched, the newly inactive
buffer is copied out to the DTrace consumer. This policy assures that the
consumer always sees a self-consistent buffer: a buffer is never simultaneously
traced to and copied out. This technique also avoids introducing a window
in which tracing is paused or otherwise prevented. The rate at which the buffer
is switched and read out is controlled by the consumer with the switchrate option. As with any rate option, switchrate may
be specified with any time suffix, but defaults to rate-per-second. For more
details on switchrate and other options, see Chapter 16, Options and Tunables.

Note –

To process the principal buffer at user-level at a rate faster
than the default of once per second, tune the value of switchrate.
The system processes actions that induce user-level activity (such as printa() and system()) when the corresponding record
in the principal buffer is processed. The value of switchrate dictates
the rate at which the system processes such actions.

Under the switch policy, if a given
enabled probe would trace more data than there is space available in the active
principal buffer, the data is dropped and a per-CPU drop
count is incremented. In the event of one or more drops, dtrace(1M) displays a message similar
to the following example:

dtrace: 11 drops on CPU 0

If a given record is larger than the total buffer size, the record will
be dropped regardless of buffer policy. You can reduce or eliminate drops
by either increasing the size of the principal buffer with the bufsize option
or by increasing the switching rate with the switchrate option.

Under the switch policy, scratch space for copyin(), copyinstr(), and alloca() is
allocated out of the active buffer.

fill Policy

For some problems,
you might wish to use a single in-kernel buffer. While this approach can be
implemented with the switch policy and appropriate D constructs
by incrementing a variable in D and predicating an exit() action
appropriately, such an implementation does not eliminate the possibility of
drops. To request a single, large in-kernel buffer, and continue tracing until
one or more of the per-CPU buffers has filled, use the fill buffer
policy. Under this policy, tracing continues until an enabled probe attempts
to trace more data than can fit in the remaining principal buffer space. When
insufficient space remains, the buffer is marked as filled and the consumer
is notified that at least one of its per-CPU buffers has filled. Once dtrace(1M) detects a single filled buffer,
tracing is stopped, all buffers are processed and dtrace exits.
No further data will be traced to a filled buffer even if the data would fit
in the buffer.

To use the fill policy, set the bufpolicy option
to fill. For example, the following command traces every
system call entry into a per-CPU 2K buffer with the buffer policy set to fill:

# dtrace -n syscall:::entry -b 2k -x bufpolicy=fill

fill Policy and END Probes

END probes
normally do not fire until tracing has been explicitly stopped by the DTrace
consumer. END probes are guaranteed to only fire on one
CPU, but the CPU on which the probe fires is undefined. With fill buffers,
tracing is explicitly stopped when at least one of the per-CPU principal buffers
has been marked as filled. If the fill policy is selected,
the END probe may fire on a CPU that has a filled buffer.
To accommodate END tracing in fill buffers,
DTrace calculates the amount of space potentially consumed by END probes
and subtracts this space from the size of the principal
buffer. If the net size is negative, DTrace will refuse to start, and dtrace(1M) will output a corresponding
error message:

dtrace: END enablings exceed size of principal buffer

The reservation mechanism ensures that a full buffer always has sufficient
space for any END probes.

ring Policy

The DTrace ring buffer
policy helps you trace the events leading up to a failure. If reproducing
the failure takes hours or days, you might wish to keep only the most recent
data. Once a principal buffer has filled, tracing wraps around to the first
entry, thereby overwriting older tracing data. You establish the ring buffer
by setting the bufpolicy option to the string ring:

# dtrace -s foo.d -x bufpolicy=ring

When used to create a ring buffer, dtrace(1M) will not display any output
until the process is terminated. At that time, the ring buffer is consumed
and processed. dtrace processes each ring buffer in CPU
order. Within a CPU's buffer, trace records will be displayed in order from
oldest to youngest. Just as with the switch buffering policy,
no ordering exists between records from different CPUs are made. If such an
ordering is required, you should trace the timestamp variable
as part of your tracing request.

The following example demonstrates the use of a #pragma option directive
to enable ring buffering: